Implantable prostheses have been used to recontour human soft tissue since at least the early 1950's. The most frequently used prostheses are breast prostheses. More than 2 million women have prosthetic breast implants and this number-continues to grow with an additional 200,000 breast prostheses implanted in the United States each year. Beisang, et al., Radiolucent Prosthetic Gel, Plastic and Recon. Surgery, 885-892, May 1991. Breast prostheses may be used for cosmetic or reconstructive purposes, for example, following a radical mastectomy or other trauma to the soft tissue of the breast. Notwithstanding the frequency of breast augmentation procedures, there are potentially severe complications associated with the implants used for such procedures.
Capsular contracture around an implant is the principal cause for dissatisfaction among implant recipients. Encapsulation is a natural response of human soft tissue to a foreign material, such as a prosthetic implant. The body perceives the implant as a foreign material which can neither be dissolved nor extruded and, in an effort to isolate the foreign material, the body forms a collagen-capsule around the implant; All implants elicit this foreign body reaction. Under normal healing circumstances, once a capsule is formed, the inflammatory reaction should cease. Persistent irritation may cause the capsule to contract around the soft implant, precipitating the development of thick fibrous scar tissue that makes the breast feel hard and painful to the touch and results in dissatisfaction and even deformity. Whalen, R. L., Connective tissue response to movement at the prosthesis/tissue interface, Biocompatible Polymers, Metals & Composites, M. Szycher, ed., Lancaster, Pa. Technomic Pub. Co., Inc., 1983, pp. 953-974.
The most common breast implant, the silicone-filled implant, comprises a hollow shell filled with silicone oil or silicone gel. Cronin & Gerow, Augmentation Mammaplasty: A New "Natural Feel"Prosthesis, Transactions of the Third International Congress of Plastic and Reconstructive Surgery, Amsterdam: Excerpta Medica, pp. 41, 1964. The leakage of silicone oil or silicone gel into the surrounding soft tissue may be caused, for example, by a compromise in the shell integrity, such as is caused by surgery or trauma. But even in the absence of such traumatic event, silicone oil continually migrates through the wall of the implant into surrounding tissue. Some of the complications associated with silicone-filled breast prostheses are related to the body's inability to eliminate the silicone oil. The silicone accumulates and a chronic inflammatory reaction may result. It has also been suggested that leakage of silicone into the surrounding soft tissue may induce auto-immune disorders.
Any of the foregoing may necessitate extensive surgery to remove the implant. But removal is not always a complete treatment. Due to the potential systemic nature of some of the complications, such complications may persist beyond such removal of the offending implant. Beisang, supra.
In addition to the foregoing, because silicone is radiographically dense, silicone-filled breast prostheses may obscure mammographic resolution. This may conceal microcalcifications and soft tissue masses and delay the detection of cancerous masses. This is quite significant when it is considered that one in nine women will eventually experience breast cancer and that cancer will recur in one of three of these women. Beisang, supra. Many of these women will have reconstructive surgery which utilizes prosthetic implants that may delay the detection of recurrence.
Despite the potentially severe complications and interference with mammographic techniques for the detection of masses, silicone-filled implants have gained widespread acceptance. This has prompted research related to improving prosthetic implants.
With respect to breast implants, several inventors have suggested that the foreign body reaction precipitated by prosthetic implants may be controlled by altering the host-prosthesis interface. Accordingly, numerous attempts have been made to change the outer shell in a manner that decreases or minimizes the foreign body reaction. For example, U.S. Pat. No. 4,955,909, in the name of Ersek, et al., discloses an implant having a shell with a textured surface. It is suggested that the textured surface will allow for fibroblast ingrowth into the interstices of the shell and thus prevent micromotion at the host-prosthesis interface thereby decreasing the amount of scar tissue formed.
U.S. Pat. No. 4,963,150, in the name of Brauman, discloses an implant shell comprising a flexible container having an outer plastic covering that is rough textured with numerous pores or interstices. It is suggested that this plastic covering may prevent the contracture of surrounding tissue.
U.S. Pat. No. 4,731,081, in the names of Tiffany, et al., discloses a rupture-resistant prosthesis. The prosthesis is rendered rupture-resistant by injecting a liquid with uniformly dispersed lubricating material into a flexible, creasable shell during manufacture or surgical implantation. The lubricating material may reduce frictional wear along opposed inner surfaces during sliding contact in a creased area in the shell wall after implantation.
Despite prior attempts, a rupture-proof or leak-proof shell has not been developed. In U.S. Pat. No. 4,795,463, Gerow disclosed an implant labeled with radiopaque markers so that roentgenographic imaging may be used to determine whether the shell has ruptured or whether the shell is folded persistently in a particular location increasing the probability that the envelope may rupture along the fold line.
Unfortunately, knowing that the shell has ruptured or is likely to rupture does not necessarily avoid the complications resulting from leakage of the filling material, for example, silicone, into surrounding soft tissue. Some inventors have suggested changing the filling material of the implants to a material less likely to precipitate such a severe adverse reaction if it should leak into soft tissue. One such filling suggested is saline. A great advantage to the saline-filled implants, as compared to the silicone-filled implants, is that the leakage of saline into surrounding soft tissue is not associated with the systemic complications that may result from silicone leakage. In addition, as compared to the silicone-filled implants, the saline-filled implants are relatively radiolucent, i.e., they do not obstruct mammographic resolution to the extent that silicone does and thus are less like to obscure detection of small masses by mammography. Yet despite these advantages, saline-filled implants have not been nearly as popular as silicone-filled implants. One disadvantage to the saline-filled implants, is that they have a 1% spontaneous deflation rate due to the prevalence of fold-flaw fractures and valve leakage. Beisang, supra. However, the lack of popularity of saline-filled implants is more readily attributable to the lack of viscosity. Simply stated, saline lacks the viscosity of silicone, and it is the viscosity which provides a consistency which more closely mimics the consistency of human breast tissue. Implant recipients want the implants to closely mimic the consistency of human soft tissue.
Inventors have searched for a more viscous implant filling material. U.S. Pat. No. 5,067,965 suggests using a radiolucent bio-osmotic gel medium filling comprising a bio-compatible organic polymer in a solution of bio-compatible salt of relatively low Z number wherein said filling is excretable by the body, has an osmolarity of from about 250 milliosmoles to 350 milliosmoles and an x-ray absorption approximately equal to that of breast tissue under standard exposure. One such material suggested by the disclosure in polyvinylpyrollidone. If the implant should rupture, the gel should preferably be removed from the body by irrigation of the tissues. The remaining gel material is removed by the reticuloendothelial system and excreted through the kidneys.
Another alternative, is a breast implant filled with a biocompatible triglyceride such as peanut oil or sunflower seed oil, as disclosed in U.S. Pat. No. 4,995,882 in the name of Destonet, et al. This implant purportedly duplicates the photoelectric interaction of fat which is both a substantial component of human breast tissue and the major producer of subject contrast at low radiation levels as used in mammography. Thus it is better radiographically than silicone-filled implants. However, the filling used in this implant, similar to saline, lacks the viscosity of silicone gel.
In the past, despite the availability of purportedly safer alternatives, silicone-filled implants were the most frequently used implants. Even after the recent FDA moratorium on silicone-filled implants and extensive adverse publicity, the use of such implants persists. There is an urgent need for the development of an implant filled with a viscous material that causes little adverse tissue reaction if it comes in contact with the surrounding soft tissue.